The use of getter materials for the removal of gaseous impurities has found application in many fields, such as process chambers for semiconductor devices, purification of process gases and pumping members for evacuated chambers. However, one of the fields in which the use of getter materials is particularly appreciated I that of the removal of harmful species from the internal volume of hermetic or sealed devices, where the presence of these harmful species jeopardizes the operation of the device.
In this case, the mechanisms that compromise the functionality of the device are essentially of two types, the first one of which is due to a chemical interaction of the harmful species with one or more components of the device, which interaction alters the properties of said components thus jeopardizing their functionality. Examples of these interactions may be, among others, some loss of transparency for optical devices or a degradation of the electrical characteristics of the components by altering their resistivity and thus their functionality. In this first case it is very important that the concentration of harmful species, typically in the form of gases, is as low as possible.
A second degradation mechanism is instead associated with the risks of breaking the device due to an excessive pressurization; this problem is present in the devices in which the harmful species are mainly in a gaseous form and their production is associated with the operation of the device itself. In this case risks of mechanical breakage of the container, and thus also safety problems, are associated with a malfunction of the device.
This problem is particularly felt in the field of electrochemical devices for energy storage, nowadays generally known as energy storage devices.
In the wide range and variety of electrochemical devices for energy storage two very important large families may be identified: electrolytic capacitors, with particular reference to those known in the field as “aluminum capacitors” and “supercapacitors”. In the technical field, the main difference between these categories of devices resides in the different order of magnitude of the accumulated capacity. In particular, in the case of electrolytic capacitors of small size the capacity is in the order of microfarad (μF), whereas in the case of supercapacitors the capacity may also be 10,000 times higher.
The problem of the presence of gaseous impurities within these devices has been tackled in various ways. For example patent applications WO 2007/066372 and WO 2008/148778, both in the applicant's name, employed polymeric multi-layer systems with the getter material dispersed in a suitable polymer and shielded from the contact and the interaction with the electrolyte by means of a protective polymeric layer permeable to the harmful species but impermeable to the electrolyte.
Another solution, described in patent applications WO 2007/080614 and WO 2008/148781, both in the applicant's name, teaches the use of getter materials enclosed in a polymeric container permeable to the harmful species but impermeable to the electrolyte.
Finally, patent application WO 2008/033560, also in the applicant's name, exploits a completely different approach and describes the use of metal getter multilayers for removing hydrogen from electrochemical devices for energy storage, with particular reference to the use of materials comprising an external layer made of a noble metal. In such application it is envisioned the possibility to use the material in powder form within suitable polymeric containers that has no other technical function than retaining the material within the container, since those material were described as compatible with the electrolytic environment of the energy storage device.
The latter solution seems, and is so considered in the field, better than the previous ones with reference to the removal of H2 because the use of polymeric multilayers necessarily limits the amount of getter material contained therein, due to the very nature of a polymeric layer; so, on equal volume occupied by the getter material, results in a lower capacity. The solutions in which the getter material is enclosed in a polymeric container have proved to be inherently fragile, in particular in the connection areas of the container. In addition to these problems there is also the fact that in the first two solutions the presence of the polymeric layer generally slows down hydrogen sorption, whereas the multilayer metal getter, which is described as such in patent application WO 2006/089068 in the applicant's name, has characteristics that were considered to make it compatible with the application.
In particular, although the solution described in WO 2008/033560 is very effective in the removal of hydrogen under normal conditions of use within electrochemical devices for energy storage, it has surprisingly shown unexpected drawbacks under some particular use conditions in which the materials intended to remove hydrogen became themselves gas sources, thus leading to the device breakage.
The main condition leading to this reverse behaviour is the presence of a flow of current having reversed polarity with respect to the normal operation. This situation may result from a human error during the connection and installation of the device, with a significant associated safety risk due to the large amounts of gas that may be generated in a short time, or when the inner temperature of the device exceeds the nominal temperature of the capacitor (which is typically defined in the field as “rated temperature” of the device), since in this case secondary alternating currents are generated (defined in the field as “ripple currents”), whose inverse component is the one which is harmful for the device. More references and details can be found in various publications such as the book Electronic Fundamentals & Applications, published in 1970. The intensity of the phenomenon, and thus the associated gas generation, is directly proportional to temperature: in particular this starts to be significant when the temperature of the device exceeds the specified rated temperature by 5%.